1. Field of the Invention
The present invention generally relates to optical disk apparatuses and, more particularly, to an optical disk apparatus which reproduces information recorded on an optical disk such as a CD-ROM, the information being recorded by using a CLV controlling method.
2. Description of the Related Art
Generally, similar to a compact disk (CD) used for audio recording, an optical disk (hereinafter referred to as simply a disk) such as a CD-ROM records digital signals according to a modulation method referred to as an Eight to Fourteen Modulation (EFM).
In the conventional CD-ROM, an interval between each bit signal and an interval between each frame signal recorded on the CD-ROM are the same on an inner side and an outer side of the disk. Accordingly, The conventional CD-ROM reproducing apparatus controls the rotational speed of the disk so that the linear velocity of an optical head relative to the disk is always constant. This control is referred to as a constant linear velocity (CLV) control method. That is, the rotational speed of the disk is varied according to a position of the optical head along a radial direction of the disk. Thus, when the optical head is moved to a target address, the rotational speed of a spindle motor must be changed to the rotational speed required at the target address from the rotational speed required at the current address of the optical head.
In the above-mentioned conventional CD-ROM apparatus, in order to achieve a high speed seek operation, a spindle motor which has a short control time for its rotational speed is required. Such a spindle motor requires a high torque and, thus, there is a problem in that the spindle motor is expensive. Additionally, since a rotational speed of the spindle motor is greatly varied during a seek operation, there is a problem in that power consumption of the spindle motor is increased.
In order to eliminate the above-mentioned problems, it has been suggested to reproduce information on the disk which was recorded by the CLV control method by using a constant angular velocity (CAV) control method. In this attempt, a synchronization clock signal derived from a reproduction signal by a reproduction PLL circuit is used as a read reference clock signal required for an operation such as an error correction after a demodulation of data. Accordingly, if a synchronization of the reproduction PLL circuit fails, it is not possible to perform an error correction after a demodulation of data. Thus, there is a problem in that error rate in the reproduced data is increased.